Project 1: In studies of IgA B cell differentiation in Peyer's patches we have obtained evidence that a major reason for preferential IgA B cell development at this site is due to enhanced survival of post-switch IgA B cells. In particular, we have shown that Peyer's patch cell populations entirely depleted of sIgA+ B cells by cell sorting technique, support the survival and differentiation of purified sIgA+ B cells, whereas spleen cell populations fail to do so. In other studies of IgA B cell differentiation, we have shown that highly differentiated IgA B cells (and IgG B cells) respond poorly to stimulation by agents that cross-link the Ig receptor (a one-signal event) but respond well to stimulation by T cells (a multi-signal event). In fact, cross-linking of IgA (with anti- IgA-dextran) on sIgA+ B cells abrogates subsequent stimulation of the B cells by T cells: anti-IgA-dextran "tolerizes" IgA B cells. Such tolerization is, however, inhibited by costimulation of cells with CD4OL. Project II: In molecular studies of B cell differentiation, we have shown that B cell-specific activation protein (BSAP) binds to a site in the 3'alpha enhancer and that such binding downregulates enhancer function as detected in studies of transfected plasmids containing the enhancer linked to a reporter gene (luciferase gene). In parallel studies we showed that downregulation of BSAP with anti-sense oligonucleotides leads to markedly decreased B cell proliferation, whereas upregulation of BSAP with BSAP- producing plasmid leads to in-creased B cell proliferation. Taken together, these results support the concept that BSAP promotes B cell growth and differentiation while it inhibits plasma cell development and Ig secretion. Project III: As part of a broad study of T cell differentiation in Peyer's patches (PP), we have studied PP dendritic cell capacity to present antigen (cytochrome C) to T cells from mice expressing a cytochrome C-specific TCR transgene. We showed that PP dendritic cells are phenotypically distinct from spleen dendritic cells (express more NHC class II) and in both primary and secondary culture systems induce T cells to produce 5-10-fold more IFN-gamma than spleen dendritic cells. This process is IL-2-dependent and inhibitable by IL-4. The capacity of PP dendritic cells to induce T cells to produce IFN-y is less evident in highly purified dendritic cells isolated by sorting on the basis of N418 antigen expression. These results indicate that PPs contain a phenotypically and functionally unique dendritic cell population that leads to TH1-type T cell responses.